Mechanisms for the modulation of alkylating activity by the quinone group in quinone alkylating agents

Previous studies have demonstrated that the quinone group may play an important role in modulating the alkylating activity of quinone alkylating agents. Introduction of a quinone moiety markedly increased the alkylating activity and cytotoxic activity of the model quinone alkylating agents benzoquinone mustard and benzoquinone dimustard. However, the cytotoxic and DNA-damaging activity of benzoquinone mustard was considerably greater than that of benzoquinone dimustard. In this study, we have investigated the role of the quinone group as a modulator of alkylating activity in these antitumor agents, using extracellular assays to eliminate differences due to cellular drug uptake and metabolism. Evidence was obtained that the alkylating activities of both benzoquinone mustard and benzoquinone dimustard were enhanced by reduction of the quinone group. In addition, when these agents were reduced, they displayed equal alkylating activity. This finding suggests that the difference in the activity of these agents in cells is not due to intrinsic differences in alkylating activities of the activated forms of these agents. Electrochemical studies revealed that benzoquinone dimustard has a lower redox potential than benzoquinone mustard and, thus, is less easily reduced. Inactivation and spectroscopic studies suggested that a major reason for the differences in activity between benzoquinone mustard and benzoquinone dimustard may be the rapid inactivation of the dimustard before its reduction. This effect may be enhanced by the lower redox potential of benzoquinone dimustard, compared with benzoquinone mustard. These findings support the hypothesis that the quinone group can modulate the alkylating activity of quinone alkylating agents; however, the mechanisms by which this modulation occurs may vary for different antitumor agents.